Gas turbines employ multi-stage axial flow compressors, which comprise as many as seventeen banks of rotating fan blades, with each circumferential row of blades followed by a row of fixed blades called stators. The compressor shaft is connected to a gas-expansion turbine such that rotation of the expansion turbine shaft causes the compressor shaft to rotate. Rotation of the compressor shaft causes the first stage blading to suction air through the compressor inlet. The air is compressed as it passes the first stage of non-rotating blades, then flows to the next stage of rotating blades where the compression process is repeated. The result at the end of all the stages of compression is air that has been compressed to about ten times higher than ambient pressure.
Axial flow compressors must be carefully controlled to avoid surge. Surge results when the pressure in the later stages of compression becomes too high. This results in a sudden reversal in the direction of air flow through the turbine which can lead to catastrophic failure of the compressor, such as blade breakage, or damage to the inlet filter and housing, or both. A full blown surge may result in hot combustion gases flowing backwards from the combustor and out the inlet of the compressor, which hot gases can also cause considerable damage to the gas turbine and associated parts.
The shaft power output of a turbine is proportional to the mass flow of air through the expansion turbine. Increasing mass flow results in more power output. But, the compressor is a constant volume machine. For a given rotational speed, the compressor has what is often referred to as a constant swallowing rate. In other words, the mass flow is determined by the geometry of the first stage blades; with each revolution they “slice off” a fixed volume of air, and that is the only mass of air introduced to the turbine.
Inlet fogging is used to increase the output of gas turbines by evaporative cooling the inlet air stream. When the air is cooler and denser the turbine makes more power than when it is hot and dry. Thus, inlet air fogging increases the amount of power produced by the turbine. Inlet fogging can cool the air stream only to the ambient wet bulb (i.e. when 100% humidity is reached, any remaining droplets will fail to evaporate).
It can also be desirable to add more water droplets in the fog than will evaporate before reaching the compressor. Since the compression process heats the air, any liquid droplets that are carried into the compressor by the air stream, will evaporate in the early stages of the compressor. This increases mass flow (since the liquid water droplet has more density than the air it displaces) and reduces the work consumed by the compressor by further cooling the air, which makes it more dense. This “intercooling” effect is variously referred to as wet compression or fog intercooling or simply overspray or overfogging.
There is a risk inherent to inlet fogging (with or without overspray) which has not, so far, been adequately addressed and mitigated by manufacturers of inlet fogging systems. If the fog is suddenly stopped, the compressor will suddenly see hotter, less dense air and the first stages of compression will undergo a significant reduction in pressure. Compressor surge may result and, in fact, has resulted in at least one case.
Fog systems are typically deployed in stages with each stage providing several degrees of cooling (or the equivalent when used for overspray.) These stages can, and often are, removed over time to reduce the possibility of compressor surge. Alternatively, fog systems are controlled by varying the pressure of the feed water, so that more or less water flows from the same number of fog nozzles.
But what about the event of sudden loss of fogging due to failure of the high pressure pumps or sudden lack of water supply? This invention provides a nearly fail-safe technique for avoiding compressor surge, which could result from the sudden loss of fogging.